Dry Casks 101: Managing Heat

In this series we’ve been talking about storing spent nuclear fuel in dry casks. One major function of these casks is to cool the fuel. Keeping the spent fuel from getting too hot is one way to ensure casks will be safe. As the fuel cools, heat is transferred from inside the cask to the outside.

Our experts look at how the cask will perform this function. We require the cask and fuel to remain within a certain temperature range. Our review looks at four main areas:

Spent fuel releases heat as a result of its radioactive decay. This is called decay heat. A key function of dry storage casks is to move the decay heat from the cask to the outside environment to ensure the fuel and cask components do not get too hot. Our experts look at how that heat will move through the cask and into the environment.

The method used to remove heat has to be reliable and provable. Heat must also be removed in a way that is passive—meaning no electrical power or mechanical device is needed. Casks use conduction, convection and radiation to transfer the heat to the outside.

The graphic shows the three heat transfer methods. As you can see, conduction transfers heat from the burner through the pot to the handle. The process of heat rising (and cold falling) is known as convection. And the heat you feel coming off a radiator, or a hot stove, is known as radiant heat.

These methods work the same way in a storage cask. Where the canister or metal structure containing the fuel touches the fuel assemblies, heat is conducted toward the outside of the cask. Most casks have vents that allow outside air to flow naturally into the cask (but not into the canister) and cool the canister containing the fuel (convection). And most casks would be warm from radiant heat if you stood next to them. (The heat generated by a loaded spent fuel cask is typically less than is given off by a home-heating system.)

We limit how hot the cask components and fuel materials can get because we want to protect the cladding, or the metal tube that holds the fuel pellets. Limiting the heat is one important way we can ensure the cladding doesn’t degrade. The cask must keep spent fuel cladding below 752 degrees Fahrenheit during normal storage conditions—a limit that, based on the material properties of the cladding, will prevent it from degrading. The fuel must also remain below 1058 degrees in off-normal or accident conditions (such as if a cask were dropped while it is being positioned on the storage pad, or if a flood or snow were to block the vents).

We also confirm the pressure inside is below the design limit to make sure the pressure won’t impact the structure or operations. Our experts review applications for new cask designs carefully to verify the fuel cladding and cask component temperatures and the internal pressure will remain below specified limits.

Each storage cask is designed to withstand the effects from a certain amount of heat. This amount is called the heat load. We look at whether the designer correctly considered how the heat load will affect cask component and fuel temperatures. We review how this heat load was calculated.

We also verify that the cask designer looked at all the environmental conditions that can be expected because these will also affect the cask component and fuel temperatures. These may include wind speed and direction, temperature extremes, and a site’s elevation (which can affect internal pressure). To make sure the right values are considered, we verify they match the historical records for a site or region.

We review all of the methods used to prove that the storage system can handle the specified heat loads. We also verify any computer codes used in the analysis and the values that were plugged in. For example, we look at the material properties for cask components used in the code. We look at calculations for temperatures and pressure. We make sure the computer codes are the latest versions.

And we only allow designers to use codes that have been endorsed by experts. We might run our own analysis using a different computer code to see if our results match the application.

The analysis and review allow us to see whether and how the dry cask will meet the temperature limits. Our review ensures the temperature is maintained and the cladding is protected. Finally, our review confirms the cask designer used acceptable methods to analyze or test the system and evaluate the thermal design. If we have any questions or concerns, we ask the designer for more information.

Only when we are satisfied that our requirements are met will we approve the thermal analysis in a cask application.

At Fukushima, one of the casks was opened and they determined the aluminum based fuel assembly baskets may not last more than 40 years. These are bolted lid metal casks, so they were able to open the cask without damaging it, unlike the thin welded canisters more common in the U.S. How will we know if the aluminum baskets used in the Holtec and other thin canisters have degraded before they fail? Currently, even the exterior of the loaded thin canisters cannot be inspected or repaired. Aging management should be built into any good engineering design. The NRC should stop approving dry storage that doesn’t have built-in aging management now that we know these systems will be used for decades if not longer. Also, the NRC should require an approved remediation plan be put in place and included in the technical documents and made public prior to any more dry cask approvals.

At Fukushima, one of the casks was opened and they determined the aluminum based fuel assembly baskets may not last more than 40 years.

The Google translation of the Japanese paper is somewhat muddled, but there appears to be no problem with the baskets falling apart per se, just that they won’t have the rated impact resistance due to the alloy grain structure changing under thermal annealing.

This only matters if the canisters are to be transported. If you can’t trust the aluminum alloy lattice grids when trucking the canister somewhere, you re-package the contents. If you don’t have to move anything, you don’t have to do anything. Or, you just move things right away so that the deterioration happens after any threat of impact has passed. Future canisters go with stainless steel and avoid the problem at the outset.

There’s a new generation of nuclear reactors coming which will consume today’s “spent” LWR fuel, either in fractions (BN-1200, S-PRISM) or whole (Transatomic). Today’s spent fuel is an energy resource twenty times as big as the energy originally extracted from it. That stuff isn’t going to be left to sit for all that much longer.

I’m not banking on future tech to do anything; I bank on physics. The heat output of today’s SNF will fall with a half-life of about 30 years no matter what anyone does. After 40 years, I doubt it will matter if the support grids collapse or not; the fuel will not overheat. What I expect is that the resource value of that fuel will not go un-tapped that long.

Donna — Great comment. I agree that the NRC should demanding “best Practice’s” instead of bargain basement solutions, especially for long term storage of ☢ Waste.

The NRC should also require that those storing ☢ Waste post a large Bond so that should there be a need in the future to deal with problems relating to the ☢ Waste there will be money available for that purpose instead of having the taxpayers foot the bill.

Think how wrong decisions about Handford and WIPP have come back to haunt US, thanks to short term cheap “solutions” that were adopted to satisfy the Nuclear Industry.

Government 101
Well, the problem with cask are…it’s a facilitative accommodation. Our nation has become too disorganized and balkanized to come up with a global solution to nuclear waste.
Mike Mulligan
Hinsdale, NH

And upon whose doorstep do you lay the blame for said “balkanization?” We have all the industrial processes proven and in place for dealing with reactor “recyclables.” France and Russia perform reprocessing every day. The DOE has been has been transporting spent military cores for years. It’s not a huge mystery what to do with the stuff….it’s a gigantic red herring.

Like the government hating southern teabaggers putting the “Mixed Oxide Fuel Fabrication Facility” on the government’s dime?
Recycling is way too expensive and dirty…
Unless they invent a miraculous process…
At least its job for Americans…

Like the government hating southern teabaggers putting the “Mixed Oxide Fuel Fabrication Facility” on the government’s dime?

Apparently the people who write the propaganda you read omitted the fact that use as MOX fuel is required by the weapons-reduction treaty, to change the isotopic composition of the plutonium to make it unusable for weapons.

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